Papermakers use mineral fillers, and other additives in the papermaking process for various reasons. Since the mid-80s, precipitated calcium carbonate, or PCC, has become the filler of choice for many papermakers—certainly in North America, but also in many other places throughout the world. There’s a simple explanation for this: PCC has proven itself to be the most functional of all possible fillers.
The performance of fillers in paper depends upon the intrinsic properties of the material, as well as the manner in which the material is used. Important intrinsic properties are refractive index, shape (or morphology), specific surface area (m˛/g), specific gravity, brightness and/or whiteness, chemical purity, surface charge particle dimensions that can be expressed as average size or size distribution, and—not least of all—relative cost.
Some of the major advantages of using PCC fillers in paper are:
- Increased sheet brightness and opacity over nearly all other fillers, due to excellent light scattering ability and intrinsic whiteness, brightness, and chemical purity of the PCC itself.
- Better formation and sheet structure that occurs as PCC fills void volumes within the sheet matrix and also increases the internal light scattering of the sheet.
- Increased sheet caliper (bulk) for morphologies such as scalenohedral PCC, which comprise particles that are not truly solid but contain a significant void volume, resulting in a relatively low effective particle density.
- Improved texture and feel that results from a smoother surface and more uniform composition. Sheet smoothness also is enhanced when sheet bulk allows for increased calendar (finishing) pressures.
- Better printability that results from a more uniform, smoother surface as well as less print show-through thanks to higher opacity and a more rapid acceptance of ink, which allows faster setting.
- Cost savings that accrue from increased filler loading (requiring less cellulose pulp fiber) in the paper, better on-machine performance leading to better machine runnability, lower demand for chemicals such as optical brighteners, lower energy demand for drying, and the intrinsically beneficial economics of PCC itself.
PCC, alone among minerals, is able to provide this unique balance of properties. Because PCC is produced by a synthetic process, no other filler can be tailored as easily to maximize the properties that are important to individual paper makers.
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